Resistive-capacitive (RC) active filters provide an economical arrangement for controlling the transmission of signals. These filters are almost always realized as a cascade of second-order sections for sensitivity and tuning considerations. T.Deliyannis in an article entitled: "High-Q Factor Circuit with Reduced Sensitivity", Electronics Letters, 1968, 4 No. 26, pp 577-579; describes an RC-active filter which uses a single differential-input operational amplifier that is suitable for realizing high-Q factor second-order band-pass functions. In a later article by the same author entitled: "RC Active Allpass Sections", Electronics Letters, 1969, 5, pp 59-60; and errata: Electronics Letters, 1969, 5, p 128; there is described two networks which realize a second-order all-pass function. Evolution of these simple negative feedback sections by J. J. Friend in a paper entitled: "A Single Operational Amplifier Biquadratic Filter Section", 1970 IEEE International Symposium Circuit Theory (Atlanta, Ga.) Dec. 14, 1970, pp 179-180; resulted in the realization of band-pass, low-pass, high-pass, band-stop and all-pass second order sections.
In constructing such filters utilizing either thin or thick film techniques, it is usually necessary to employ laser, abrasive or anodization trimming of the resistive components to achieve the final functional parameters of the filter. Consequently, it is advantageous to be able to use a unidirectional tuning sequence by trimming only resistors. If the trimming elements are all interdependent as is the case with Friend's circuits, it will be necessary to utilize nominal capacitors of very tight tolerance and then to individually trim each resistor to its nominal value in a series of ever-decreasing steps in order not to overtrim; or to measure the capacitive values and then utilize a computer to calculate the desired values of the resistors assuming a nominal operational amplifier characteristic prior to trimming each individual filter. This involves the added complication of having to provide test points in the integrated circuit for measuring the resistors and capacitors, and also of connecting the components back into the circuit once the measurements are made. Thus, RC-active filters of the type described by Friend above, do not permit functional tuning simply by trimming the resistive components in a predetermined sequence while monitoring the overall magnitude and/or phase of the filter. While active filter circuits which provide unidirectional functional tuning have been developed by others in the past, in general they do not have the structural simplicity or the low sensitivity of the above-mentioned circuits.